AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Required practical activity 2: investigate the effect of antiseptics or…
Full specification wording
Culturing microorganisms (biology only): Required practical activity 2: investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition.
Culture bacteria on a sterile agar plate and place discs soaked in antiseptic or antibiotic on the surface.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 9 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Culture bacteria on a sterile agar plate and place discs soaked in antiseptic or antibiotic on the surface. The substance diffuses into the agar. Where effective, bacteria do not grow, leaving a clear area called a zone of inhibition. After incubation, measure the full diameter of each clear zone in millimetres (including the disc), usually in two perpendicular directions, and calculate a mean. A larger zone indicates greater effectiveness. Control variables include the volume and concentration of each substance, the bacterium, incubation temperature, and disc size. A control disc soaked in sterile water proves the clear zone is caused by the substance. Work aseptically: sterilise equipment, keep lids secured, and do not open plates after incubation.
What earns the marks
- Places discs soaked in known volumes of antiseptic or antibiotic on an inoculated agar plate using aseptic technique.
- Includes a control disc soaked in sterile water to show the effect is due to the test substance.
- Controls key variables such as disc size, volume and concentration of solution, bacterium used, and incubation temperature and time.
- Measures the full diameter of each zone of inhibition in millimetres, taking two perpendicular measurements and calculating a mean.
- Compares zones to judge which substance is most effective, recognising that a larger zone means greater inhibition of growth.
- Handles plates safely, sealing and incubating them appropriately and not opening them after incubation.
Common mistakes and how to put them right
Measuring only the clear area from the edge of the disc to the edge of the zone.
Put it right: Measure the full diameter of the clear zone, including the disc itself.
Using different volumes or concentrations for each substance.
Put it right: Keep these the same so the comparison is fair.
Omitting a control disc.
Put it right: Include a water-soaked disc so any clear zone can be attributed to the test substance.
Exam tips
- State the independent variable, dependent variable and at least two control variables when describing the method.
- Explain why the plate is not opened after incubation, linking this to avoiding contamination.
- When comparing results, quote the zone measurements and use them to justify which substance is most effective.
Try the lesson
The start of the Culturing microorganisms (biology only): Required practical activity 2: investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition. lesson, made from checked teaching notes for this specification point.
Have a go first
In the required practical, why is a zone of inhibition measured around a disc soaked in antibiotic?
Listen
Culture bacteria on a sterile agar plate and place discs soaked in antiseptic or antibiotic on the surface. The substance diffuses into the agar. Where effective, bacteria do not grow, leaving a clear area called a zone of inhibition. After incubation, measure the full diameter of each clear zone in millimetres (including the disc), usually in two perpendicular directions, and calculate a mean. A larger zone indicates greater effectiveness. Control variables include the volume and concentration of each substance, the bacterium, incubation temperature, and disc size. A control disc soaked in sterile water proves the clear zone is caused by the substance. Work aseptically: sterilise equipment, keep lids secured, and do not open plates after incubation. A strong answer earns marks for these points. Places discs soaked in known volumes of antiseptic or antibiotic on an inoculated agar plate using aseptic technique. Includes a control disc soaked in sterile water to show the effect is due to the test substance. Controls key variables such as disc size, volume and concentration of solution, bacterium used, and incubation temperature and time. Measures the full diameter of each zone of inhibition in millimetres, taking two perpendicular measurements and calculating a mean. A common slip: Measuring only the clear area from the edge of the disc to the edge of the zone. Instead, measure the full diameter of the clear zone, including the disc itself. Exam tip: State the independent variable, dependent variable and at least two control variables when describing the method.
Key words
Say what each word means before you turn the card.
zone of inhibition
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control disc
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aseptic technique
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control variables
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See every definition
- zone of inhibition
- A clear area where bacteria do not grow, left where an effective antiseptic or antibiotic diffuses into the agar.
- control disc
- A disc soaked in sterile water, used to prove that the clear zone is caused by the test substance.
- aseptic technique
- Working safely by sterilising equipment, keeping lids secured, and not opening plates after incubation.
- control variables
- The volume and concentration of each substance, the bacterium, incubation temperature, and disc size, kept the same for a fair comparison.
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